The present invention generally relates to electrical relays. More specifically, the present invention relates to such relays having a piezoelectric film.
Relays are used to switch a wide range of electrical circuits by using solid state or mechanical switches that permit and prevent electrical components from receiving power supply. Relays may perform satisfactorily in some applications, but often suffer some unfavorable performance characteristics, such as being unreliable, slow, large, costly, and/or power consuming. These unfavorable performance characteristics produce unreliable circuits and prevent the use of one specific relay in many different applications.
It is recognized that a relay may perform satisfactorily in a variety of circuits, applications and environments if certain beneficial features are achieved. Many of these features include high power load capability, low power switch actuation, minimization of arcing, cheap and simple fabrication, and small size. For example, relays with high current-carrying capability are needed for circuits in various satellite and cellular communications equipment. Furthermore, smaller, lower power relays are desirable for the ever shrinking size of circuitry in consumer electronics and other commercial and non-commercial applications. Although a variety of conventional relay designs attempt to achieve these and other beneficial attributes, conventional relays still suffer some undesirable characteristics.
For example, solid-state switches based upon semiconductor technology such as GaAs field effect transistors (FETs) may be utilized to construct an electrical relay. Such a relay is small in dimension and provides fast switching at low operating voltages. However, a solid-state relay can only switch circuits carrying small currents (e.g., in the micro-amp to milliamp range). Furthermore, due to the semiconductor nature of these FET devices (i.e., the p-n junctions), a solid-state relay generally possesses high on-resistance, which generally means that the impedance of power flow through the relay is relatively high.
Relay devices based on mechanical actuators are also known, including those driven by electrostatic, magnetic, thermal, hybrid, and bulk piezoelectric actuators. Because these switches are operated by mechanically connecting and disconnecting two metal contacts, mechanical relays often benefit from lower on-resistance than solid-state relays.
Electrostatic relays are actuated by applying an electrostatic field between two electrodes to move a thin, flexible film. These relays can be micromachined to very small sizes, and can produce relatively sizable displacements at relatively high frequencies to ensure an open circuit in an off condition. However, due to thin metal electrode layers that are generally used for the metal contacts, the current carried over the contacts is limited to the milliamp range. Furthermore, high voltage is typically required for electrostatic relay operation (e.g., 50-100 volts). Magnetic relays, like electrostatic relays, also provide large displacements between the metal contacts. Unlike electrostatic relays, however, magnetic relays can carry relatively large currents (milliamp to amp range) over the electrical contacts, and can produce very strong contact force between the circuit contact elements. Nonetheless, magnetic relays generally suffer from a number of disadvantages, including magnetic operation, short lifetime and large size. Thermal buckling relays also provide high force actuation and large displacement, but typically require high power consumption. Moreover, switching speeds are often slow in these actuators, and the thermal power required to drive thermal actuators of these thermal buckling relays can cause deleterious effects on circuit performance.
A hybrid drive relay is another type of mechanical relay, utilizing a combination of actuating devices. Conventional hybrid drive relays include hybrid thermal/electrostatic and piezoelectric/electrostatic actuators that combine the attributes of higher force actuation (e.g., thermal or piezoelectric) with higher displacement actuation (e.g., electrostatic actuation). The thermal/electrostatic drive, however, still requires thermal power for actuation which can cause deleterious effects on circuit performance. The piezoelectric/electrostatic drive uses the natural expansion and/or contraction of a piezoelectric material under an applied electric field in combination with an electrostatic drive to actuate one or more contact elements.
One such hybrid piezoelectric/electrostatic relay is described in U.S. Pat. No. 5,666,258, to Gevatter et al. Gevatter et al. discloses a hybrid micromechanical relay including both a piezoelectric actuator and an electrostatic actuator, where the two actuators operate in tandem to supply a relatively strong contact force between contact elements. The hybrid piezoelectric/electrostatic relay, however, utilizes the piezoelectric element merely to assist the electrostatic actuation by providing higher force. The thickness and geometry of the piezoelectric layer is not optimized such that the piezoelectric is the primary driver. Furthermore, this hybrid drive requires multiple processing steps on separate wafers for each driver element and also requires wafer bonding for final assembly. This method complicates device fabrication compared with monolithic micromachined structures.
Piezoelectric materials also may be exclusively utilized as actuating means in bulk piezoelectric relays, another type of mechanical relay. An example of a bulk piezoelectric relay is described in U.S. Pat. No. 4,595,855, to Farrall. Farrall discloses a piezoelectric relay formed by adhesive bonding assembly steps using a ceramic piezoelectric material. Like other types of mechanical relays, a piezoelectric bulk ceramic relay suffers from some weaknesses. The bulk ceramic relay is relatively large in dimension, and would likely be used as a discrete component added separately to a circuit to be switched, or machined into a bimorph or array structure. The relay can also require high operating voltages for actuation, in the range of several hundred volts.
Generally, therefore, an unsatisfied need exists in the industry for a low cost and easily fabricated relay with a high force output that allows for thick metal contacts for switching of high currents, fast switching speeds, long lifetime and relatively low operating voltages.
These and other advantages are provided, according to the present invention, by providing a miniature electrical relay having a thin film piezoelectric actuating element as the primary driver. A miniature electrical relay in accordance with the present invention can take numerous forms to provide desired results. For example, the present invention can be embodied in the form of a cantilever relay or a bridge relay. Such miniature relay advantages include cheap and simple fabrication, low operating power, high force output, high current switching capability, and integration with small circuits.
In accordance with the present invention, a piezoelectric actuating element is deposited on a movable contact of a miniature relay. The piezoelectric actuating element includes a piezoelectric material, preferably a piezoelectric film with a thickness in the range of 1-10 xcexcm, sandwiched in between two metal electrode layers that function as piezoelectric electrodes. The piezoelectric actuating element can be actuated by applying an electric field across the metal electrode layers, such as by selectively connecting the metal electrode layers to a positive terminal and a negative terminal, respectively, of one or more power sources. The metal used to construct the metal electrode layers should be platinum, silver or some other metal that may withstand processing conditions of the piezoelectric material, which may be in the range of 500-700xc2x0 C. in an oxidizing atmosphere.
The piezoelectric actuating element is affixed to a movable circuit contact, mounted upon a support structure such as a substrate or integrated chip. The movable circuit contact is deformable with respect to a fixed circuit contact mounted on another portion of the support structure. In a cantilever relay according to the present invention, the movable circuit contact has one fixed end and one movable nonfixed end, whereas in the bridge relay according to the present invention, the movable circuit contact has at least two fixed ends, with a movable central portion.
According to one aspect of the invention, when the piezoelectric actuating element is actuated, the movable circuit contact deforms to selectively engage one or more fixed contacts. According to an alternative embodiment, when the piezoelectric actuating element is actuated, the movable circuit contact deforms to disengage one or more fixed contacts. In either embodiment, when the movable circuit contact is mechanically and electrically coupled to the fixed circuit contact(s), the relay is in a conductive state. When the movable and fixed circuit contact(s) are separated by a gap, the relay is in a nonconductive state.
In order to provide electrical isolation between the metal electrode layers of the piezoelectric actuating element and the contacts, an insulating layer such as silicon oxide or silicon nitride is deposited between the metal electrode layers and the circuit contacts. The thickness of the insulating material will depend on the voltage of the circuit to be switched, as the insulating layer should provide electrical isolation between the piezoelectric actuating element and the circuit contacts.
In the cantilever relay, larger displacements with smaller device dimensions can be achieved but with lower force capability due to the free movement on the non-fixed end of the piezoelectric actuator. The bridge relay requires larger device dimensions and produces smaller displacements but generates higher piezoelectric forces than the cantilever design. The device dimensions of the relay may be between 100 micrometers to a few millimeters in length, although other dimensions may be achieved. Because the piezoelectric element is flexible, the cantilever or bridge will generally be in planar configuration over the fixed contact when the relay is in a nonconductive state. However, the cantilever and diaphragm may be designed to have a natural recoiled or non-planar configuration when the relay is in a nonconductive state. The strong actuation force of the piezoelectric actuating element allows for relatively thick metal electrode layers to be utilized for the circuit contacts, allowing the relay to carry large currents.
Due to its micromachined construction, piezoelectric relays according to the present invention can be integrated with the circuit to be switched, and electrical signals from the circuit contacts and metal electrode layers can be easily isolated to prevent arcing between them by providing a sufficient insulating layer between them. Relays according to the present invention also use less power and can be fabricated in large quantities more efficiently by wafer-scale processing compared with discrete bulk relay devices. Finally, relays according to the present invention are smaller and use less power than other relays, and provide a wide temperature range of operation due to the high Curie temperatures (as high as 300-400xc2x0 C.) of the piezoelectric thin film material.
Other features and advantages of the present invention will become apparent to one skilled in the art upon examination of the following drawings and detailed description. It is intended that all such features and advantages be included herein within the scope of the present invention as defined by the appended claims.